Corrosion-resistant aluminum alloy article



GIGS,

Patented May 12, 1931 UNITED STATES PATENT OFFICE runners ,c. FBARY,

or omens, rnnnsynvanm, rassrenon 'ro runnunrnnu cou- PANY OF AMERICA, 01 PITTSBURGH, PENNSYLVANIA, A CORPORATION OF PENN- SYLVANIA CORROSION-RESISTANCE- .ALUMINUM ALLOY ARTICLE No Drawing.

Aluminum base alloys are for many purposes superior to pure aluminum, especially the so-called strong alloys, but these alloys are in general subject to attack by corrosive agenand the corrosion thus caused results in a loss of tensile strength and elongation. This corrodibility militates seriously against the use of the alloy in certain fields,'as for example in aircraft, where failure of even a single art may mean disaster. For the purpose o overcoming the defect it has been proposed to provide articles of strong alloy with a coating of pure or substantially pure aluminum which has high resistance to corro- I agencies.

sion, and alloy thus protected, sold under the name Alclad, has gone into extensive use.

Aluminum has, however, low tensile strength in comparison with the alloys referred to, and hence an alloy article protected by an aluminum coating or layer has less strength, weight for weight, than an unprotected article of the same alloy. Where for an adequate margin of safety the protective coating isa substantial proportion of the total weight the difference in strength may be an important item, amounting in some cases to as much as 10 per cent. Moreover, aluminum is a soft metal, far softer than the alloys referred to, and hence a coating of aluminum gives the article a surface which is not adapted to stand abrasive or erosive Need has therefore been felt for protected articles of greater strength and surface hardness, at the same time possessing the desired resistance to c0rrosi0n,- and I have accordingly been led to devise my present invention,

which has for its chief object the-provision of an article of strong alloy, with a protec-i tive coating of another strong alloy which may be hardened by suitable thermal processing and possesses the necessary corrosionaesistance. y

The strong aluminum alloys require heattreatment to develop their desirable properties to the fullest-extent. .This treatment usually consists in heatingthe finished article to an elevated temperature, say around 500 (1, quenching to alowtemperatureby means of a cooling medium; andthenffaging'the heated and ficial aging, which consists in heating the: alloy to a temperature between approximatev .ly 100.

per cent copper, 0.5 per cent. magnesium,

num alloy, containing magnesium Application filed November 30, 1929. Serial No. 410,877.

alloy. Aging of the quenched alloy may be effected in'diiferent ways, depending upon the nature of the alloy. Some aluminum alloys age spontaneously, atroom temperature, .and develop full strength without further treatment, while others do not age in that way or do so only in small degree. case of ,these latter alloys, if it is desired to fullydevelop their advantageous physical properties they must, after they have been quenched, be subjected to arti-l and 150 C. Alloys which age at room temperature and alloys which require artificial aging can be conveniently described as spontaneously ageable and ageable, respectively. I

'In general, alloys which age spontaneously at room temperature arenot as to corrosion attack as are thosexwhich have been artificially aged. I On the other hand, alloys which have undergone aging as-part of their heat-treatment generally have better physical properties than when aging is omitted. I have found, however, that .an aluminum alloy containing about 0.7 per cent of magnesium and about 0.9 per cent In the susceptible of sill? artificially con, which is one of the alloys capable of artificial aging, possesses, in the quenched. condition without aging, its maximum resistance to corrosion andat the same time has the I hardness, and tensile strength required for my purpose. on the otherhand, an alumi num alloy containing approximately 3. to 5.5

0.6 per cent manganese, ages spontaneously. I therefore use the latter alloy (aluminums copper-magnesium -manganese) as the base or core of-my article and provide it with aproand tective coating of the firstsmentioned alumiand silicon. Then when the article is heated and i quenched I have an article consisting of a base-orcore composed of an alloy which ages atroom temperature, and. a p v spontaneously,

coating alloy which does not age.spontanestrength, so that the.

characteristic of resistance to corrosion but also a de ree of hardness and tensile strength only slig tly less than that which would be the alloy-coated metal, after being subjected; -to mist from a spray of 20 per cent saltsolution for two years, showed a tensile strength of 58,180 pounds per square inch, as against 59,260 pounds before the test, the loss of strength being roughly 1.8 per. cent. This was the greatest loss sustained, the lowest being less than 0.2 per cent. Themetal used for the coating also exhibits a Brinell hard- .ness of 64, and isthus well suited to uses where abrasionand scratch-resistance is important. In the test referred to, the elongation decreased less than '7 .5 per cent and the yield point 6.6 per cent.

In making the article in the preferred way an ingot is made by casting the base alloy against the protective alloy, which latter may be in the form of plates or sheets. The ingot is then subjected to heat and pressure, as by rollin and intermediate heating, to complete the initial bond produced by the casting, the working being continued to produce a sheet a or plate of the desired thickness for fabrication.

It is convenient to arrange the protective alloy plates or sheets in the form of a lining for the mold in which the ingot or other 7 article is to be cast, but it is not necessary,

and indeed it is not in eneral desirable, to have all inner surfaces 0 the mold lined with the plates. On the contrary it usually is sufiicient and in most cases advisable to line only two opposite surfaces, thou h in casting ingots or billets for drawing tu es, rods, wire, etc., the entire inner surface of the mold may have a lining in the form of a tube, which is to be considered the equivalent of one or more plates or sheets.

In order that the ingot may be rolled down or otherwise shaped by rolling, drawing, or like operations, the casting of the base alloy against the alloy mold-lining must produce an initial bond which will withstand shearing or similar stresses produced in such opera-' tions tending to cause the surface plates to slip on the -un initial bond without harmfully affecting the surfacing sheets against which the alloy is cast it is advisable to observe certain precautions.

In the first place the tendency of the erlying alloy. To insure thismolten alloy to fuse ordis solve the surfacing sheets should be restricted so that the fusion or solution produced is only superficial. A certain amount of diffusion of one metal into the other is often desirable and in some cases essential, but this diffusion can be produced by subsequent heating and working of the ingot, and if the initial diffusion or penetration of components of the base alloy extends too close to the outer surface of the sheets'or plates of the protective alloy the subsequent diffusion, combined with the thinning of the protective layer by working, may in effect result in converting such layer lIltO a corrodible alloy. While care in rollor other working, and avoidance of too high temperatures in heating the ingot for working, will aid in preventing the result alluded to, care in such operations is not always suilicient and hence the casting operation should be properly carried out. It is therefore desirable to use a mold which will conduct, absorb, or dissipate heat rapidly, and the lining sheets should be in close contact therewith, so that the heat imparted to the sheets or plates by the larger body of molten alloy, preferably at atemperature as low as possible consistent with proper casting and initial bonding, will flow to the mold walls too rapidly to permit more than surface fusion of the aluminum. Iron molds having smooth inner surfaces have been found satisfactory for the purpose, and these are preferably water-cooled. In.- some cases it may-be desirable or even necessary to heattreat the protective alloy plates, or make them of heat-treated alloy, without, however, artificial aging. This preliminary heat treatment puts in solid solution those alloy constituents which form with the aluminum a low-melting eutectic, thus eliminating (by such solution) as much as possible of the low-melting eutectic.

' Even with molds of the kind just described it is advisable to use lining plates of substantial thickness not only because too thin a plate is more easily melted or raised to a dissolving or diffusing temperature but also if the plate is not of sufiicient thickness originally the subsequent reduction in thickness by working may make it too thin to afford the desired degree of protection.

It is also advantageous, in casting the ingot, to avoid pouring the molten alloy against any part of the mold lining, as it has been found that a stream of molten alloy flowing on the plate is'apt to make a hole or a cavity therein. Such effect may be due in part, at least, to erosion or some analogous action. Avoidance. of splashing of the molten alloy is also desirable. In general I try to have the alloy fold itself, so to speak, into contact with the plates, as a, li uid poured gently into a vessel rolls or folds into contact with the walls. Then if there is any substantial fusion of the alloy plate the fused portion is not moved out of place, and dee er and more extensive fusion is therefore ess likely to occur. A good method of casting is to pour the molten alloy in a gentle stream down an unlined surface of the mold, tilting the mold at .first so as to incline the unlined as the pouring proceeds. In

surface and gradually straightening it up this way, with the temperature of the alloy, the mass of the alloy and the mass of'the lining sheets, and therate of pouring so related that the alloy freezes against the lining surface at about the same rate 'as disadvantageous conditions mentioned can be avoided.v What I aim at is to produce a union which is predominantly one of co hesion as distinguished from. adhesion.

Contrary to expectation, my observation is that the surface oxidation ordinarily found on the protective alloy does not prevent the necessary initial bonding, and hence, al-

though dirt and grease should, obviously, be

removed, 1t has not been necessary to clean the allo by etching with acid or alkali, or

- 'merous points or minute areas of cohesion are extended or spread out, but intervening oxide particles are not. Hence in the aggregate the area of molecular cohesion so greatly exceeds the aggregate area of the island-like non-cohering points that the latter are in effect non-existent. Whether the theory just outlined be true or not, a union comparable to such a bond in completeness and tenacity is obtainable when the operation is carried out I in grooves.

with care and intelligence.

. The coating plates may be held against the inner surfaces of the mold by means of clamps, or they may have their edges seated Making the lining in two or more parts is advantageous in that it permits expansion with less or no tendency to buckling or warping during heating by the hot alloy. In many cases annealing the coating material minimizes such tendency.

The initial bond the alloy core is made complete by heat and pressure, preferably rolling pressure, re-

peated as may be necessary to bring about complete union and,

ingot or billet to a plate or sheet of the de-- if desired, reduce the sired thickness. In such treatment there is more or less diffusion of components of the cast alloy into the coating plates, and care should be taken not to cause or permit this alloying action to extend too close to the its contact therewith, the.

of the surfacing plates toouter surface. It is therefore desirable in most cases to .keepthe working temperature below that at which too rapid solution of copper or. other non-aluminous metalor metalspresent takes place. For this reason, especially in the .case of alloys whose physical properties areto be improved by heat-treatment, it is desirable that the temperature to which it is heated for rolling or otherworking should be Well belowthe melting point of the eutectic or eutectics. In rolling I have observed that there is greater tendency for the coating alloy to rise or lift with the. rolls than-is the case where pure aluminum is used as, the protective metal, but this tendency can be overcome by using lighter passes or less reduction per pass.

As stated above, the preferred protective alloycontains approximately 0,7 per cent magnesium and 0.9 per cent silicon, but these proportions can be varied, remembering, however, that in general the larger the amounts of the ingredients named the harder the alloy is to work. The alloys should not contain copper beyond what may be present as an inconsequential impurity, but should contain enough magnesium and silicon to ive an effective amount of the compound g Si. It may contain one or more other metals in small amount, such as nickel, manganese,chromium and titanium, which tend to increase the hardness of the alloy without seriously impairing its resistance to corrosion. The base or core alloy is preferably one containing 3 to 5.5 per cent of copper, 0.5 per cent magnesium, and 0.6 per cent manganese, with or without other ingredients sometimes used in aluminum-copper alloys. The proportions named can be varied, and the-manganese can be omitted entirely, with some decrease of tensile strength'inthe heat-treated article. So also the magnesium, but the omission of this elementdestroys or at least impairs the capability of spontaneous aging and is not recommended.

In speaking of heating and working the ingot after castingI'donot mean to imply that the ingot must be allowed to cool below the working temperature and then reheated. On the' contrary it is only'when the casting has cooled, too far that reheating need be 1 resorted'to', and in some cases it is advantageous to begin the working without allowing the casting to cool far enough tonecessitate reheating. Thisis'especially true with' corealloys of high magnesium content, or, in gen eral, with alloys which oxidize readily. With such alloys,

points air g I result that in the]reheating'thefoxide oa-niit appears that if the con: I and the coatingsheets do not stick at all finds its way in between,1'with the tride film, or'wliatever it maybe on thecast- '1 ally increased" and may" becometo perinit eventual'attainnient of a" However, if the ingot be subjected to pressure, say a light pass between rolls, before .it hascoo'led too far preferably as'soon as it can be" taken from the mold and before it has cooled appreciably the union can be improvedto such an extent that an adequate initial bond can be obtained.

I do not claim the process herein described, but what I claim as my invention is:

1. A corrosion-resistant article of aluminous metal comprising a main body or core of aluminum-copper alloy and a hard, wrought, protective coating of copper-free aluminum alloy cohesively united with the core, thecoa-ting alloy being resistant to -corrosion after heat-treatment without artificial aging, and both alloys being susceptible of substantial improvement in physical properties by heat-treatment without artificial agmg.

2. An article composed of corrodible spontaneously ageable aluminum alloy of high tensile strength and a hard protective coating of corrosion-resistant artificially ageable aluminum alloy of high tensile strength cohesively united with the core and containing the compound Mg- Si, both alloys having the physical properties characteristic 4 thereof when heat-treated without artificial aging.

3. A corrosion-resistant aluminuous metallic article comprising a mainbody or core of spontaneously ageable aluminum-copper alloy cohesively united with the core and containing magnesium, and a hard corrosion-resistant coating of artificially ageable copperfree aluminum alloy containing magnes um and silicon, the article having been heattreated without artificial aging as evidenced by a tensile strength exceeding about 55000 pounds" per square inch and a Brinell hardness not less than about 60, the article'also being capable of withstanding prolonged exposure to mist from a 20 per cent salt solution without material loss of tensile strength.

sp'ective alloys a nous metal, comprising a main body or core of alumlnum alloy contaming, approximately, 3 to- 5.5 per cent copper, 0.5 per cent magnesium, and 0.6 per cent manganese, and a hard, cohesively joined coating of coppercontaining magnesium and silicon, cohesivelyunited to the main body and possessing greater, resistance to corrosion before artificial aging than after; both the main body or core and the coating of the article having the physical pro erties produced in the re ter heat-treatment without artificial aging.

In testimony whereof I hereto aifix my sig-I nature.

FRANCIS o. FRARY.

4. A heat-treated corrosion-resistant aluminous metallic article" comprising a main body or core of readily'corrodible spontaneous ag'eable aluminum-copper alloy containmg magnesium and-manganese and a 0.0-

hesively joined coating of copper-free corrosion-resistant aluminum alloy containing magnesium and silicon and at least one other constituent 'ada ted to have a hardening effect upon the al oy.

5. A corrosion-resistant article of aluminous metal, comprising a main body or core of spontaneously ageable aluminum alloy containing, approximately, 3 to 5.5 per cent copper, 0.5 per cent magnesium, and 0.6 per cent manganese, and a hard cohesively joined coating of copper-free corrosion-resistant artificially. ageable aluminum alloy containing, approximately, 0.7 .per cent magnesium .and 0.9 per cent si con.

6. A corrosion-resistant articleof alumi- 

